1. Field of the Invention
This invention is directed to a rubber composition particularly formulated for use in tread portions for automobile tires.
2. Description of the Prior Art
Automotive tires are usually required to be safe and economical to run and comfortable to ride. To cope with the advent of highway networks, tire qualities have grown more and more strict with respect to cornering and braking characteristics, at high-speed running, among many other important properties. To this end tire treads are generally formed from rubbers of high hysteresis loss associated with friction or gripping on the road surface.
Hysteresis loss is generated by periodic deformation of the tire tread while in high-speed, frictional contact with even slight undulations on the road. The more frequently the tread dissipates energy at its surface areas contacting with the road, the greater frictional force. This tread deformation is known to depend, as evidenced by the William-Landel-Fery superposition principle of temperature against time, upon the magnitude of hysteresis loss measured at a lower temperature, say in a temperature difference of 30.degree. to 40.degree. C., than that at which the tire is run. More specifically, the loss factor of the tread, called tan .delta. and taken as a measure of the hysteresis loss, has been found fairly correlative with the friction coefficient of the tire at that temperature.
To attain great hysteresis loss imparted to the tire tread, it has been proposed that certain styrene-butadiene rubbers (SBR) be used which are abundant in styrene content and hence high in glass transition temperature (T.sub.g). A limited region of temperatures dominates the friction quality of the tire because rubbery material reaches a peak of tan .delta. in the vicinity of T.sub.g corresponding to 0.degree. C. when the tire runs at 30.degree. C.
In the loss factor-temperature curves emulsion-polymerized SRR rubbers tend to displace their peaks of tan .delta. toward a high temperature side as the styrene contents increase, providing great tan .delta. at the foot of the curve and at around 0.degree. C. This leaves the problem that tan .delta. will depend widely upon ambient temperature and thus render tire gripping objectionably variable. Such rubbers have further drawbacks in that they show a sharp rise in elastic modulus at a low temperature side and thus fail to follow a rough road and in that on ice and snow they become wholly undeformable, leading to unacceptable cornering and braking qualities.
Rubbers of low T.sub.g temperatures, typified by butadiene rubbers (BR) of great cis contents, are known to excel in gripping at low temperature but only to an extent to invite insufficient tan .delta. at about 0.degree. C. and hence inadequate gripping at high temperature.
To resolve the problems encountered with the above SBR and BR rubbers, many attempts have been made by blending SBR with BR or by adding large amounts of particulate carbon black as disclosed for instance in "The Friction of Penumatic Tyres", D.F. Moore, Elasevier Scientific Publishing Co., 1975, U.S. Pat. No. 4,748,168 and Japanese Patent Laid-Open Publication No. 62-12937.
Known SBR-BR blends are prone to get converged at a single peak in the plots of loss factor against temperature. This means that the two rubbers have their peculiar benefits offset, leading to insufficient gripping at both high and low temperatures. Too much carbon black will in most instances render the resulting rubber mix susceptible to heat buildup.
Japanese Patent Laid-Open Publication No. 61-66733 and No. 62-62840 teach incorporating a certain low-temperature plasticizer in an SBR-BR blend. This additive, though efficient in improving gripping at low temperature, is liable to markedly decline in modulus at high temperature and hence instable runnability. Such sort of dilemma is attributed primarily to the viscoelastic behaviors of SBR and BR rubbers used. Reformed rubbers of SBR and BR, therefore, have been proposed which are produced by the use of an organic lithium catalyst.
Motor vehicles are run usually in various environments. In one example roads vary in surface temperature in the order of tens of degrees. To enhance gripping over such a wide range of temperatures, tread rubber should preferably be high in temperature and broad in width at a peak of tan .delta.. Block polymers are known to show a broad peak of tan .delta. as disclosed for instance in Japanese Patent Laid-Open Publication No. 57-102912 and No. 57-109817 in which block polymers are formed to have two different SBR blocks of varying styrene and 1,2-bond contents. With peak width in view, however, these prior polymers are not broad enough to meet the above temperature range and oftentimes dependent upon the combination of blocks and thus likely to displace their peak temperatures toward a high or low temperature side. There is nothing in the latter two publications to suggest that such block polymers should be resistant to skidding on ice.
Japanese Patent Laid-Open Publication No. 54-62248 discloses that BR rubbers of high 1,2-bond contents may provide a good balance between wet gripping and rolling resistance. BR of this type is necessarily inadequate with respect to gripping at low temperature as T.sub.g is rather high.
In Japanese Patent Laid-Open Publication No. 57-109817 and No. 57-108142 there are taught SBR copolymers made up of a high T.sub.g block of SBR having a styrene content of 20 to 50% by weight and a 1,2-bond content of 40 to 75% by weight and a low T.sub.g block of SBR having a styrene content of less than 10% by weight and a 1,2-bond content of 20 to 50% by weight. Such copolymers have improved wet gripping and rolling resistance.
Japanese Patent Laid-Open Publication No. 57-102912 is directed to an SBR copolymer composed of two blocks, one block of 10 to 50% in styrene content and of 20 to 50% in 1,2-bond content and the other of 1 to 30% and of more than 60%, respectively, in similar contents. This copolymer, because of the blocks differing in solubility parameter from each other, shows two T.sub.g temperatures in an uncured state and on vulcanization becomes compatible at a single T.sub.g temperature, thus developing a broad peak in the plot of loss factor against temperature.
It is known that a broad peak of tan .delta., hence a good balance among wet and icy gripping, rolling resistance and breaking strength, can be provided by taking advantage of a wide distribution of vinyl contents with a T.sub.g difference of two blocks set at above 30.degree. C. Cited in this respect is Japanese Patent Laid-Open Publication No. 60-192739. One of those blocks is an SBR block of 10 to 80% in styrene content and of 30 to 70% in vinyl content, and the other is a BR block of less than 60% in vinyl content.
Japanese Patent Laid-Open Publication No. 61-55135 teaches balancing the properties of a block copolymer by adjusting in its weight average molecular weight-number average molecular weight ratio to 1.8 to 5.0, which copolymer appears in No. 60-192739 above. This molecular weight adjustment leads to low an absolute value of tan .delta..
A copolymer, taught by Japanese Patent Laid-Open Publication No. 61-231016, is made up of three SBR blocks. Such prior copolymer, though adequate in icy and wet gripping, is still insufficient with respect to the level of quality improvement.